Post-transcriptional regulatory mechanisms play a role in gene expression in probably all organisms. TRAP of Bacillus subtilis regulates tryptophan biosynthesis and transport by transcription attenuation and translational control mechanisms. When activated by tryptophan, TRAP binds to a 5'stem-loop (5'SL) and 11 (G/U)AG repeats in the nascent trpEDCFBA operon leader transcript, thereby promoting transcription termination before RNA polymerase can reach the trp operon structural genes (attenuation). TRAP binding also promotes formation of an RNA structure that inhibits trpE translation. NusA-stimulated RNA polymerase (RNAP) pausing participates in the attenuation and trpE translation control mechanisms. In addition, TRAP binding to multiple triplet repeats in the pabA (trpG), trpP (yhaG) and ycbK transcripts regulates translation by directly blocking ribosome binding. These mechanisms will be further analyzed to gain a more complete understanding of how B. subtilis regulates tryptophan metabolism, to strengthen our knowledge about the diversity of post-transcriptional control mechanisms, and to determine some of the fundamental principles that dictate protein-RNA recognition. Since several human disorders are caused by sequestration of triplet repeat RNA-binding proteins, combined with the findings that expression of HIV and several oncogenes is regulated by attenuation, results from these studies will indirectly contribute to improving human health. The mechanism of RNAP pausing and its role in attenuation and trpE translation control will be examined using a combination of genetic and biochemical approaches. The mechanism of TRAP-5'SL interaction will be investigated by determining the amino acid residues of TRAP that interact with the 5'SL and the 5'SL nucleotides that interact with TRAP. In addition, experiments will be performed to determine if the 5'SL serves as an mRNA instability determinant. The mechanism of TRAP-mediated ycbK translation control will also be examined. While the TRAP-dependent translation control mechanisms for pabA, trpP and ycbK are similar, it is apparent that the relative arrangement of the triplet repeats results in different degrees of TRAP-mediated control. Genetic and biochemical experiments will be carried out to understand how the relative placement of the triplet repeats influences TRAP-mediated translation control of these three genes.